This invention relates to spark plugs and more particularly to spark plugs which includes precious metal electrode inserts and a method of making electrodes therefor to improve durability.
Spark plugs in internal combustion engines, igniters in gas turbines and jet engines are used for igniting combustible mixtures of gases and vapors. The electrodes for these devices are generally made of nickel base alloys, nickel alloy/nickel clad copper core, precious metals (platinum, Palladium, Iridium, etc.), or a combination of these materials. For economic reasons, the demand for longer life devices has been increasing, and the industry has responded with various proposals for extended life spark plugs and igniters. In the past, such devices were made by using precious metal electrodes. Since these metals are expensive, a compromise between service life and cost was reached by use of combination nickel base alloy electrodes and precious metal inserts.
Electrode materials experience quite severe environmental conditions, such as thermal cycling, pressure cycling, vibrations, electrical discharge cycling, exposure to deposits, and exposure to low and high temperature reactive gases. In designing electrodes, as well as electrode materials, many parameters and material properties must be considered simultaneously. Examples of such parameters can include:
(i) oxidation resistance (ii) carburization resistance (iii) sulfidation resistance (iv) resistance to lead and other oil deposits (v) spark erosion resistance (vi) thermal shock resistance (vii) high temperature and pressure stability of both the base material and its protective oxides (viii) softening and melting points (ix) electrical resistivity (x) thermal diffusivity
Attempts to provide more durable spark plug and igniter construction have included several suggestions which combine use of a nickel base alloy and a precious metal insert constructions.
Examples of patents which show spark plugs and igniters with such precious metal inserts includes U.S. Pat. Nos. 3,984,717; 4,427,915; 4,465,952; 4,488,081; 4,540,910; and U.K. Pat. No. 2,005,649A. Other patents which disclose use of a nickel base heat resistant composition in the electrodes of such spark plugs and igniters and which provide thermal expansion compensation are set forth in U.S. Pat. Nos. 4,581,558; 4,659,960 and 4,670,684.
While such spark plugs are acceptable compromises, it has been found that nickel base alloys are susceptible to sulfur attack. Lead will attack along the grain boundaries and reduce electrode life.
Precious metal electrodes, while providing excellent overall properties, are far superior in spark erosion resistance as compared to nickel base alloys. Since precious metals are very expensive, the tendency is to use the minimum amount required to give an acceptable performance. As a result, small pieces of precious metal or inserts have been used at the sparking areas. Generally, these "buttons" are resistance welded to nickel base alloys.
Since the button material often differs in composition from the electrode material, differential expansion can cause cracks which can result in separation of the buttons from the electrode. Attempts have also been made (U.S. Pat. No. 4,540,910) to match thermal expansion of precious metal inserts to the base metal by intermediate layers of another material. Since these electrodes provide more useful life, they are usually referred to as "Extended Life Plugs".
Even though such extended life plugs provide improved durability compared to the standard plugs (using nickel base electrodes only), failure problems still persist. The failure mode of these plugs involves:
(a) Cracking and separation (e.g., falling) of precious metal insert from the base electrode (usually made of nickel base alloys).
(b) Oxidation of the precious metal - nickel base interface.
(c) Attack of the interface area by lead and other oil deposits.
(d) Sulfur attack of the nickel base electrode itself.
The failure mode (a) results from thermal expansion mismatch between the material of the nickel base electrode and the material of the precious metal button. Precious metal grain growth in the insert can be produced by high temperature cycling during spark plug operation. Such grain growth also contributes to the cracking problem. Finer grain material is generally more resistant to such crack induced failure (platinum electrodes normally contain very fine zirconia which pins the grain boundaries and thereby slows down the grain growth). Oxidation of the insert/electrode base material interface area (between precious metal button and nickel base electrode) can produce brittle oxide materials which further contribute to the crack failure mode.
Lead and oxide penetration at the interface area can weaken the insert to electrode bond. Eventually such penetration can cause the precious metal button to separate from the base alloy material of the electrode. High sulfur fuel use can lead to severe attack of nickel base electrode at its grain boundaries. Low melting sulfur-nickel liquid can form at operational temperatures of the electrodes. Such liquidous material form can wet such grain boundaries. Such wetting is analagous to loose mortar and bricks and can cause a sudden separation of the insert electrode (just like bricks can be separated when loose mortar fails).